Datasheet MAX7219, MAX7221 (Analog Devices) - 10

MAX7219/MAX7221 Serially Interfaced, 8-Digit LED Display Drivers If the scan-limit register is set for three digits or less, are cascaded, then to write to the fourth chip, sent the individual digit drivers will dissipate excessive amounts of desired 16-bit word, followed by three no-op codes (hex power. Consequently, the value of the RSET resistor must 0xX0XX, see Table 2). When LOAD/CS goes high, data is be adjusted according to the number of digits displayed, latched in all devices. The first three chips receive no-op to limit individual digit driver power dissipation. Table 9 commands, and the fourth receives the intended data. lists the number of digits displayed and the corresponding maximum recommended segment current when the digit
Applications Information
drivers are used.
Supply Bypassing and Wiring Display-Test Register
To minimize power-supply ripple due to the peak digit The display-test register operates in two modes: normal driver currents, connect a 10μF electrolytic and a 0.1μF and display test. Display-test mode turns all LEDs on by ceramic capacitor between V+ and GND as close to the overriding, but not altering, all controls and digit registers device as possible. The MAX7219/MAX7221 should be (including the shutdown register). In display-test mode, 8 placed in close proximity to the LED display, and connections digits are scanned and the duty cycle is 31/32 (15/16 for should be kept as short as possible to minimize the ef ects MAX7221). Table 10 lists the display-test register format. of wiring inductance and electro-magnetic interference. Also, both GND pins must be connected to ground.
Table 9. Maximum Segment Current for Selecting RSET Resistor and 1-, 2-, or 3-Digit Displays Using External Drivers
The current per segment is approximately 100 times
NUMBER OF DIGITS MAXIMUM SEGMENT
the current in ISET. To select R
CURRENT
SET, see Table 11. The
DISPLAYED
MAX7219/MAX7221’s maximum recommended segment
(mA)
current is 40mA. For segment current levels above 1 10 these levels, external digit drivers will be needed. In this 2 20 application, the MAX7219/MAX7221 serve only as controllers for other high-current drivers or transistors. 3 30 Therefore, to conserve power, use RSET = 47kΩ when using external current sources as segment drivers.
Table 10. Display-Test Register Format
The example in Figure 2 uses the MAX7219/MAX7221’s
(Address (Hex) = 0xXF)
segment drivers, a MAX394 single-pole double-throw analog switch, and external transistors to drive 2.3”
REGISTER DATA
AND2307SLC common-cathode displays. The 5.6V zener
MODE D7 D6 D5 D4 D3 D2 D1 D0
diode has been added in series with the decimal point LED because the decimal point LED forward voltage is Normal typically 4.2V. For all other segments the LED forward Operation X X X X X X X 0 voltage is typically 8V. Since external transistors are Display Test used to sink current (DIG 0 and DIG 1 are used as logic Mode X X X X X X X 1 switches), peak segment currents of 45mA are allowed even though only two digits are displayed. In applications
Note: The MAX7219/MAX7221 remain in display-test mode
where the MAX7219/MAX7221’s digit drivers are used
(all LEDs on) until the display-test register is reconfigured for normal operation.
to sink current and fewer than four digits are displayed, Table 9 specifies the maximum allowable segment
No-Op Register
current. RSET must be selected accordingly (Table 11). The no-op register is used when cascading MAX7219s Refer to the Continuous Power Dissipation section of the or MAX7221s. Connect all devices’ LOAD/CS inputs Absolute Maximum Ratings to calculate acceptable limits together and connect DOUT to DIN on adjacent devices. for ambient temperature, segment current, and the LED DOUT is a CMOS logic-level output that easily drives forward-voltage drop. DIN of successively cascaded parts. (Refer to the Serial Addressing Modes section for detailed information on serial input/output timing.) For example, if four MAX7219s www.maximintegrated.com Maxim Integrated │ 10